First, a Mobile IP (hereinafter called an MIP) disclosed in non-patent document 1 will be described. MIP is a technique that enables a mobile communication apparatus (a Mobile Node: hereinafter called an MN) to attain movement transparency in a layer 3. According to the MIP, each MN has a home network, and employs, as a home address, a global address allocated on the home network. The home network is at least one network, or more, allocated for the MN that employs the MIP, and an assigned address on the home network is called a home address. When the MN is moved and the network to which the MN is to be connected is switched to a different network (hereinafter, called an external network), one other than the home network, the home address is still used as an address for a communication party (a Correspondent Node: hereinafter called a CN) to identify the MN on the Internet.
On the other hand, in a case wherein the MN is moved and the network to which the MN is to be connected is changed to an external network, the MN employs, as a Care-of Address (hereinafter may also be called a CoA), a global address assigned on the network. The care-of address is an address that the MN temporarily uses while on the network to which it has been moved. The MN employs a Binding Update message to register, with a home agent present on the home network of the MN, relative correlation information concerning the care-of address and the home address. The home agent function is to receive, as a proxy, a packet that has been forwarded to the home address of an MN that has registered a care-of address, and to transfer the packet to the care-of address that has been registered. Therefore, a CN that transmits a packet to the MN need not be aware that the MN is currently moving, and must merely transmit the packet, using the home address as the destination, for the packet to be properly delivered to the MN that is present on the external network.
When the MIP is employed in this manner, as well as in a case wherein the connection destination for the MN is an external network other than the home network, communication with the CN is enabled using the home address, as though a connection to the home network has been established. However, in a case wherein the MIP is employed, a problem encountered is that for a period, beginning with the disconnection of the MN from the network used prior to moving and continuing until the connection of the MN to the network at the movement destination and the re-enabling of communication using the home address, i.e., a period continuing until the transmission of the care-of address for the network at the movement destination has been completed, the MN can neither transmit nor receive packets. During this period, the home agent receives, as proxy, packets addressed to the home address of the MN; however, since the transfer destination is still the care-of address that was used on the network from which the MN was moved, the MN can not receive these packets, and as a result, the packets are abandoned. In order to resolve this problem that is encountered in a case wherein the MIP is employed, use of a technique called fast handovers for MIP (Fast Handovers for MobileIPv6: hereinafter called FMIP), which is described in non-patent document 2, is well known.
The FMIP technique will now be described by employing FIG. 14.
An access router (AR) 21, an access point (AP) 22 that is connected to the access router 21, an access router 31, and an access point 32 that is connected to the access router 31 are illustrated in FIG. 14. The access routers 21 and 31 are routers for managing subnets 23 and 34, which are subnetworks that include APs as subordinates, and for performing the routing, to an external network, of a packet transmitted by a node that is present on a subnetwork, and for the routing of a packet from the external network to a node in a subnetwork.
The AP 22 forms a radio coverage area 23, and the AP 32 forms a radio coverage area 33. By moving, an MN 10 can be connected, via the AP 22 or 32, to a subnetwork provided by the AR 21 or 31, and can also be connected to an IP network 15. The AR 21 and the AR 31 communicate with each other via the IP network 15. The connection and disconnection of an MN relative to the AP is performed under the control of a layer 2 (data link layer). In a case wherein the MN is present in the radio coverage area of the AP, the MN is connected to the AP on the layer 2, and thereafter communicates, via the layer 3, with the AR to which the AP is connected, so that the generation of an address and the setting of a default router are performed.
The operation of the FMIP will now be described by employing FIG. 14. Assume that an MN 10, in its initial state, is connected to the AP 22 and belongs to the subnet 24 formed by the AR 21. An explanation will be given for an operation in a case wherein the FMIP is performed when the MN 10, in this state, moves along a path from the radio coverage area 23 toward the AP 32, through an overlap area 25 between the radio coverage areas 23 and 33, and to the radio coverage area 33. It should be noted that hereafter there is a case wherein the AR 21, the access router before the MN 10 was moved, is called a PAR (a Previous Access Router), and the AR 31, which is the access router after the MN 10 has been moved, is called an NAR (a New Access Router).
A sequence applicable to when the MN performs the FMIP is shown in FIG. 15.
When the MN 10 has entered the overlap area 25, the MN 10 receives a beacon transmitted by the AP 32 (S101). And the identifier for the AP 32, included in the received beacon, is obtained and an RtSolPr (a Router Solicitation for Proxy Advertisement) message that includes this identifier is transmitted to the AR 21 (S102). Upon receiving this message, the AR 21 transmits to the MN 10 a PrRtAdv (Proxy Router Advertisement) message containing information for an AR (in this case, information for the AR 31) that has, as a subordinate, an AP that includes the identifier included in the message (S103).
When the handover to the AR 31 has been determined, the MN 10 employs the prefix for the AR 31, obtained through the PrRtAdv message, and configures a New Care-of Address (hereinafter, this may be called an NCoA) to be used at a destination (S104). Then, the configured, new care-of address is added to a Fast Binding Update message (hereinafter called an FBU message), and the FBU message is transmitted to the AR 21 (S105).
The AR 21 adds to a Handover Initiate message (hereinafter called an HI message), the new care-of address that is included in the FBU message, and transmits the HI message to the AR 31 (S106).
Upon receiving the HI message, the AR 31 determines whether the new care-of address included in the message is appropriate, adds the result to a Handover Acknowledgement message (hereinafter called an HAck message), and transmits this messages to the AR 21 (S107).
Upon receiving the HAck, the AR 21 adds to an FBAck message the examination result for the new care-of address that is included in the HAck message, and transmits the FBAck message to the MN 10 (S108). After the AR 21 has transmitted the FBAck message, the AR 21 receives, as a proxy, a packet that is delivered using the Previous Care-of Address (hereafter, may be called a PCoA) that the MN 10 used on the subnet 24, and transfers the packet to the new care-of address (S109).
The AR 31 performs buffering for the packet transmitted to the new care-of address of the MN 10 (S110).
When the MN 10 has performed the layer 2 handover and has been connected to the AP 32 (S111), the MN 10 transmits a Fast Neighbor Advertisement message (hereinafter called an FNA message) to the AR 31 (S112). Upon receiving the FNA message, the AR 31 transfers to the MN 10 the packet that is buffered at S110 (S113).
There are two operating modes for the FMIP: a case described above wherein, prior to the handover, the FBU message is transmitted and the FBAck message is received is called the predictive mode; and the other mode is called the reactive mode wherein, following the move, the FBU message is transmitted, via the AR 31, to the AR 21. When the FMIP is employed in this manner, the MN 10 can receive, without any loss, a packet that has been delivered during a period extending from the start of the layer 2 handover and continuing until the completion of the performance by the mobile IP at the destination. However, a problem here is that packet reception is stopped during the layer 2 handover. In a case, however, wherein the MN includes a plurality of interfaces, the above described problem can be solved.
An MN that includes a plurality of interfaces will now be briefly described. An MN having a plurality of interfaces can be a terminal that has two interfaces, such as a connection interface for a wireless LAN conforming to the IEEE802.11a/b/g and a cellular connection interface for W-CDMA, UMTS, etc. Currently, such a terminal has already appeared on the market, and its use will spread, as a more common item in the future.
The merit involved in including both a wireless LAN interface and a cellular interface is that the area covered by the cellular network is broader than the area covered by the wireless LAN. Thus, when the MN is moved outside the coverage area of the wireless LAN, or has performed a handover to the connection point of a different wireless LAN, a case will probably occur wherein the cellular interface will always be present in the coverage area of the cellar network.
Further, not only for a mode wherein a wireless LAN interface and a cellar interface are included but also for a case wherein interfaces for various other connection methods are employed, a case could occur wherein, when one of the interfaces is moved outside a coverage area or when a handover is performed, the other interface will be present in the coverage area and a connection will be continued. Furthermore, since a wireless LAN has a broader band than has a cellar network, and since the access fee will be less expensive, it is assumed that users will request to use the wireless LAN in preference to the cellar network.
Even when an accompanying situation involving the disconnection or interruption of a connection occurs, i.e., when one of a plurality of interfaces included in the MN falls outside the coverage area, or the handover is performed, the other interface will acquire a continuous connection state. In this state, the FMIP is performed for the interface moved outside the coverage area, or for the connection of the interface that performs the handover, so that regardless of whether there is a disconnection or an interruption of the interface, packets can be continuously received. This method is the one whereby the MN employs, as a new care-of address to be transmitted with a FBU message, an address assigned to the other interface that is currently connected.
As a result, when the PAR has encapsulated a packet and has transferred it to the care-of address indicated in the FBU message, the MN can receive the packet of this address through the interface that is currently connected, so that buffering performed by the MAR is not required. That is, even when one of the interfaces is currently performing a layer 2 handover, a packet delivered during this period can be received using the other interface.
Non-Patent Document 1: Johnson, D. B., Perkins, C. E., and Arkko, J., “Mobility Support in IPv6”, RFC3775, June 2004.
Non-Patent Document 2: Rajeev Koodli, “Fast Handover for Mobile IPv6”, draft-ietf-mipshop-fast-mipv6-03.txt, Oct. 25, 2004.
However, as described above, in a case wherein an MN having a plurality of interfaces employs the normal FMIP and uses, as a new care-of address, the address of a different interface to which currently connected, an AR used before moving (a Previous Access Router; hereafter called a PAR) will not be aware that the address indicated in the notification is the address of the different interface of the mobile node that is currently connected. Therefore, in the normal FMIP process, a packet delivered to the PCoA is continuously transferred to the NCoA until the entry held by the PAR is timed-out. That is, the MN has merely changed the interface to be used, and the state of the interface that is performing the handover is not considered at all.
Therefore, when the above described method is employed for a case wherein the cellular interface is the one that the mobile node transmitted to the PAR as a new care-of address, and wherein the wireless LAN interface is the one via which the handover is actually currently being performed, the packet transfer destination continues to be the cellular interface even when the handover using the wireless LAN has been completed. That is, the mobile node can not effectively employ the WLAN interface, for which the handover has been completed and the broadband connection means has been obtained.
Furthermore, not only in the above described case involving the use of the wireless LAN and the cellar interfaces, but also in a case involving the use of various connection modes, the current FMIP does not consider the fact that the interface for which the handover has been completed is to be employed again, while taking into account, for example, the communication quality state, the cost and the communication speed.
In addition, as the common FMIP process for permitting the NAR to confirm whether a new care-of address transmitted by the MN is appropriate, the PAR transmits an HI message, and receives an HAck message as a reply. Generally, the NAR for the FMIP is an access router that is present in a subnet at a new connection destination for the interface of the MN that performs a handover. However, in a case wherein, as described above, the MN transmits an FBU message by employing, as a new care-of address, an address that is allocated to a different interface to which currently connected, the PAR identifies, as the NAR, the access router that is present in the subnet for which the address is valid.
However, for the PAR, there is no guarantee that the NAR is an access router present in the neighborhood, and it is highly probable that the NAR of the MN will not be included in information for the AR available for the PAR. In this case, a phenomenon that hinders the performance of the FMIP could occur, e.g., the PAR could not know the destination address of an HI message, or even if the destination address is found, the PAR could not receive an HAck message because the access router is not compatible with the FMIP.